Polyvinyl chloride (PVC) resins are used in large quantities throughout the world. Typically these PVC resins are produced by one of three processes, suspension, emulsion or mass process. Emulsion produced PVC is very small in particle size such as 1 micron or less and is essentially non-porous. The emulsion process uses a relatively large amount of surfactants to make the small particle resins. PVC resins produced by the mass process are much larger, having a weight average particle size of from about 100 to 300 microns. The mass resin is produced in a process which normally does not use water and surfactants. The mass resin does not have a pericellular membrane around the particle. The absence of the pericellular membrane offers some advantages such as a purer product and faster uptake of plasticizer for a given porosity value of a resin, although mass resins are known for their low porosity.
By far the largest volume PVC resin is produced by the suspension process. Suspension produced PVC resins have about the same particle size and the same end uses as does mass produced resin. In the suspension process, surfactants such as cellulose materials or partially hydrolyzed acetate which is usually referred to as polyvinyl alcohol (PVA) are used to suspend vinyl chloride monomer droplets in water and the polymerization takes place in the monomer droplet usually aided by a free radical initiator.
In the suspension polymerization of PVC, droplets of vinyl chloride monomer 30-150 microns in diameter are dispersed in water by agitation and aided by surfactants. A thin membrane is formed at the water to monomer interface by dispersants such as PVA. This membrane has been measured at 0.01 to 0.02 micron thick, and has been found to be a graft copolymer of polyvinyl chloride and the dispersant(s). Early in the polymerization, particles of PVC deposit onto the membrane from both the monomer and the water sides forming a skin 0.5-5.0 microns thick that can be observed on grains sectioned after polymerization. Primary particles which are about 1 micron in size deposit on the skin from the monomer side and water phase polymerized PVC at about 0.1 micron in size deposits on the water side of the membrane.
In suspension polymerization, droplets of polymerizing PVC, 30-150 microns in size, agglomerate to form particles or grains of 100-200 microns in diameter. With one droplet per particle the shape is quite spherical. When several droplets agglomerate to form one particle, the shape can be quite irregular and knobby sometimes referred to as popcorn shaped.
Suspension produced PVC particles desirably have voids in the particle which is known as porosity. These voids (porosity) form when the various components of the particle agglomerate such as the primary particles and droplets. Porosity is a very important property of the resin because it not only allows the polymerized resin to be easily stripped of its residual monomer but also gives the resin the ability to absorb large quantities of plasticizers.
Not only is porosity important but also the particle shape is important. Spherical particles have several attributes such as fast extrusion and high bulk density. However, an agglomerated highly irregular particle has better ability to absorb plasticizer quickly because of its irregular shape.
When PVC particles are desired for a particular application which requires a "skinless" or "low skin" resin, then mass produced resin is usually the resin of choice. A previous suspension process was developed and is disclosed in U.S. Pat. No. 3,706,722 to Nelson, et al, which produces resin particles having "low skin" features. The term "low skin" as used herein refers to resin particles having less than 50% of their surface area as PVC as measured by ESCA technique. This process is known as a phase inversion process. In the early part of the polymerization, the monomer is the continuous phase and after about 10% conversion additional water is added such as to make the water the continuous phase and the monomer the discontinuous phase. This process in essence runs a mass type polymerization up to about 10% conversion and then inverts to a suspension type polymerization. The resultant resin particle, however, is a porous spherical shape particle and not the more desirable porous agglomerated irregular shape particle. Also, the prior inversion process results in very heavy polymer buildup on the internal surfaces of the polymerization equipment. This buildup is sometimes referred to as reactor fouling and is a highly undesirable feature.
It would be very desirable to have a PVC resin particle which would be skinless, highly porous, friable, have fast plasticizer uptake and have an agglomerated shape.